1. Field of the Invention
The present invention relates to an analog circuit technology. More specifically, the invention relates to a circuit technology to decrease an effect of variation of current characteristics of transistors.
2. Description of the Related Art
In recent years, a display device having a thin film transistor (TFT) formed on a glass substrate is widely used. For example, a liquid crystal display (LCD) having in each pixel a TFT formed by using amorphous (non-crystalline) silicon is in widespread use in a notebook personal computer or a portable device and the like.
The TFT formed by using amorphous silicon, however, has low mobility. Therefore, it cannot flow much current. In view of the aforementioned, a TFT formed by using polycrystalline silicon is formed on a glass substrate instead. By using the polycrystalline silicon TFT that has high mobility, it is possible to integrate a driver circuit on the glass substrate. A driver circuit is typically implemented with a digital circuit, however, it is also under study to implement an analog circuit as well aiming at the realization of System-on-Panel in which various circuits are mounted on a glass substrate.
A structure of a source follower circuit as an example of the analog circuit is described now. FIG. 21 shows a circuit diagram of a source follower circuit. An input voltage Vi is inputted to a gate terminal 4308 of a transistor TR1. A bias voltage Vb is applied to a gate terminal 4309 of a transistor TR2. A voltage between the gate and source of the transistor TR1 is denoted as Vgs1. It is assumed for simplicity that a potential of a power supply on the low potential side (Vss) is 0 V. Then, a voltage of a source terminal 4310 of the transistor TR1 (an output voltage Vo) satisfies the following formula (1).Formula 1 Vo=Vi−Vgs1  (1)
It is also assumed for simplicity that current characteristics and sizes (gate length L and gate width W) of the transistors TR1 and TR2 are identical here. The transistors TR1 and TR2 are connected in series, therefore, the same amount of current flows through each transistor. In the case where the transistors TR1 and TR2 both operate in a saturation region, the voltage Vgs1 between the gate and source of the transistor TR1 is equal to a voltage between the gate and source of the transistor TR2, that is the bias voltage Vb. Therefore, the following formula (2) is satisfied.Formula 2 Vo=Vi−Vb  (2)
Although the sizes (gate length L and gate width W) of the transistors TR1 and TR2 are designed to be identical, actual sizes often vary when fabricated. Further, variation of gate insulating films in thickness or variation of crystallinity of channel formation regions lead to variation of the current characteristics of the transistor, for example threshold voltage or mobility.
It is assumed here as an example that a threshold voltage of the transistor TR1 is 2 V and that of the transistor TR2 is 3 V because of the variation. It should be noted that the transistor flows a current, which corresponds to a value that deducted a threshold voltage from a voltage between the gate and source thereof. In order that the transistor TR1 may flow the same amount of current as that flowing through the transistor TR2, a voltage between the gate and source of the transistor TR1 becomes 1 V lower because the threshold voltage thereof is 1 V lower. As a result, the output voltage Vo becomes 1 V higher, which is defined by Formulas 1 and 2 as compared to the case where the threshold voltage of the transistors TR1 and TR2 are equal.
As described above, the output voltage Vo varies when the current characteristics or the sizes of the transistors TR1 and TR2 vary.
In view of the aforementioned, a technology for making compensation for the variation is suggested. For example, a source follower circuit in which variation of transistors is compensated is reported (refer to Non-Patent Document 1).
FIG. 24 shows a circuit diagram of the aforementioned source follower circuit. The operation thereof is described now. First, switches 4401, 4406, and 4404 are turned ON among switches 4401 to 4406. Note that a switch becomes conductive when turned ON. An input voltage Vi is applied to an input terminal 4407. Subsequently, the switches 4401 and 4406 are turned OFF and the switch 4402 is turned ON. Then, a first offset voltage is stored in a capacitor 4409. Next, the switches 4402 and 4404 are turned OFF and the switch 4403 is turned ON. Then, a second offset voltage is stored in a capacitor 4410. As a result of the aforementioned operations, variation of the output voltage Vo is compensated.